Pattern analyses of the vocal structure of dugong calls

Abstract

Dugongs (Dugong dugon) produce different types of vocalization such as chirps, trills, and barks. Previous reports showed that dugongs have two kinds of phonemes : long duration calls (trills) and short duration calls (chirp-squeaks hereinafter called chirps). However, the function of these calls was not revealed. The objective of this study is to classify the vocalization patterns of dugong calls and discuss the functional use of call sequences among a local population of dugongs. We recorded the underwater sound for 120 continuous hours by an automatic underwater sound recording system conducted in the southern part of Talibong Island, Trang, Thailand. We analyzed the underwater sound data sets that were obtained by the system. Call durations differ among local populations. Short duration calls with less than 300 ms were defined as chirps and trills were defined as a call lasting over 300 ms. The end of a call sequence was defined as a silence over 1 second. A total of 1559 audible calls were detected from a total of 15 hours data set (from 3:50 to 6:50 29 February 01-04 March 2004). Chirps were observed more than trill calls (1470 chirps and 89 trills). Chirp-to-chirp transitions were most frequency observed (83.70%), whereas trill-to-trill transitions were the least (3.97%). Transitions between the two types of calls were also observed (6.80%, 5.53%). The stability of the sequence of each type of calls in a call sequence is investigated. The call sequence analysis as well as the behavioral context observation will provide the key to interpret the function of dugong calls.